The present invention is directed to an apparatus which will adaptively tune to a received periodic signal and, in one embodiment, will identify the frequency of the received signal to a user.
Adaptive tuning to a received periodic signal is useful in a variety of situations, such as tuning to a homing beacon or other similar navigational device, or, generally, tuning to or identifying any periodic signal of unknown frequency with speed and precision.
Prior art devices for identifying and tuning to an unidentified (i.e., the frequency is unknown) received signal involved filtering the received signal through an array of high Q filters, and establishing a frequency range within which the received frequency is propagated by noting which of the high Q filters in the array passes the received signal. In order to provide a high degree of accuracy in ascertaining the frequency of the received signal, such prior art circuits must necessarily have a large number of high Q filters in the filter array. A greater number of filters reduces the interval between detectable frequencies and, therefore, increased the accuracy by which the received frequency can be determined.
Thus, such prior art devices necessarily require that some estimation of the frequency of the received signal be known in advance in order that the frequency range detectable by the filter array can be assured to include the frequency expected to be received. In order to search a very wide frequency spectrum, it is sometimes required that a plurality of prior art devices be employed, with each device addressing a different range of frequencies.
The prior art devices thus described are slow to operate, require foreknowledge of the range within the anticipated frequency of the received signal will fall, and require the use of analog devices, such as high Q filters, which are known by those skilled in the art to be less stable than digital circuit elements, especially when subjected to variations of ambient temperature.
It is desirable that an apparatus be provided for detection of frequency of a received signal which is stable in operation in a variety of environments, which is capable of accurate determination of the frequency of the received signal, and which is capable of ascertaining the frequency of the received signal speedily.